Single crystal garnets



Filed Aug. 25, 1959 Om. ON

/NVE/vmf? J. W N/ELSEN A TTORNEY United states Patent 3,650,407 SINGLECRYSTAL GARNETS llames W. Nielsen, Berkeley Heights, NJ., assignor toBell Telephone Laboratories, Incorporated, New York, N.Y., a corporationof New York Filed Aug. 25, 1959, Ser. No. 836,008 7 Claims. (Cl. 106-42)This invention relates to a method of growing single crystals ofsynthetic garnets in a ilux comprising lead oxide and lead uoride, andalso to the garnet crystals so produced.

The synthetic garnet materials of particular interest can be representedby the formulas where O is oxygen, and A and B are trivalent metals.

In particular, A may be yttrium or one of the rare earth elements ofatomic number between 62 and 71, or mixtures of rare earth elements ofatomic number between 57 and 71 with each other or with yttrium. B maybe a trivalent metallic ion such as iron, gallium, aluminum or scandium,or mixtures thereof with each other or other metallic ions. Where A isyttrium and B is iron, the garnets are known as yttriumdron garnets. Thesynthetic materials described are termed garnets, because they have thesame complicated cubic structure as the mineral garnets, such as, forexample, grossularite,

The synthetic garnets containing a substantial iron content areferrimagnetic and can be used in the construction of inductive devicesas cores therefor. Such garnets show the property of Faraday rotationfor microwaves, and in transparent section can also be used to rotatevisible light. Because of their magnetic properties, the coefficient ofspecic rotation in the garnets is larger than in most other transparentmaterials, making them especially useful.

As is well known in the art, single crystals of ferrimagnetic materialhave certain magnetic properties not exhibited by the material inpolycrystalline form. 4In particular, the resonance lines of singlecrystal materials are much narrower than those found in thepolycrystalline material, this property forming the basis for the typesof microwave devices described in copending applications Serial No.778,352, tiled December 5, 1958, now Patent No. 3,016,495 and Serial No.774,172, led November 17, 1958, now Patent No. 3,013,229. A convenientprior art method of producing such single crystals consisted ofcombining the reactants in proper proportions with a ux consisting oflead oxide, heating the mixture to form a homogeneous liquid, andforming the single crystals from the molten bath by standardcrystallization procedures. This technique is discussedin detail incopending application Serial No. 655,995, filed April 30, 1957, now U.S.Patent 2,957,827.

The present invention embodies the same general procedures as theaforementioned crystal growing method with the exception of the fluxemployed. The present inventive method utilizes a flux initiallycomprising lead iluoride or mixtures thereof with lead oxide. The use ofsuch a flux is advantageous in several respects.

An important advantag, from an economic viewpoint, of the use of a fluxcomprising lead uoride, is the substantial increase in the yield ofsingle crystal garnets expressed in terms of yield per unit weight ofmelt. Thus, for example, in the production of yttrium-iron garnet singlecrystals, the use of a ilux containing percent by weight of leadfluoride and 80 percent by weight of lead oxide produced an amount ofgarnet which was approximately 50 percent greater than that resultingfrom the use of a linx consisting of 100 percent lead oxide.

The quality and size of the garnet crystals produced by the presentinventive method are both improved over those produced by the prior artmethod. Although the largest single crystals of yttrium-iron garnetproduced in accordance with the prior art method were of the order of 2grams, crystals as large as 20 grams have been produced by the presentmethod. vIn addition, the number and size of inclusions in the crystalsare substantially reduced.

As discussed more fully below, the addition of lead iluoride to theprior art rflux simplies the separation problem in the instances wherethe garnet crystals being produced are magnetic. Thus, for example, inthe production of yttrium-iron garnet by the prior art process, crystalsof magnetoplumbite (PbFelzOlg) are formed along with the primaryproduct. This necessitates magnetic separation of the crystal atdilferent temperatures to take advantage of the diierent Curie points ofthe two magnetic materials. The addition of lead fluoride to the lluxmakes possible the adjustment of the proportions of the reactants sothat the formation of magnetoplumbite is avoided while maintaining theyield of the desired garnet at substantially high levels.

An important general advantage of the use of a flux containing leadoxide and lead fluoride for the production of rare earth single crystalgarnets inheres in the fact that the growing process may be conducted atlower temperatures than heretofore possible without sacrificing theyield. The solubility of the reactants in the flux increases at highertemperatures and, accordingly, the yield of single crystal garnets alsoincreases with temperature. A practical limitation on the maximumtemperature of the process is approximately 1400" C., since above thistemperature the melts very actively attack the platinum crucibles inwhich they are prepared. Since other-materials commonly used in Cruciblemanufacture, such as ceramics, are attacked even more readily than isplatinum, no evasion ofthis practical limitation as yet appearsfeasible. Heretofore, production of rare earth garnet crystals usingpure lead oxide as the ilux was generally conducted at temperatures asclose to 1400 C. as Y practicable in order to increase the solubility ofthe rare earth oxide reactants to the highest possible level. Thisresulted in substantial deterioration of the platinum crucibles. Theaddition of lead fluoride to the prior art flux substantially increasesthe solvent power of the ilux with respect to the rare earth oxidereactants. The increase in solubility is suiciently great to permit adecrease in the maximum temperature of the process and still maintain ayield advantage over the prior art process.

'I'he inventive flux is especially suitable for the production of a newseries of synthetic gems based on the yttrium-gallium garnet structure.The color of these gems is controlled by adding small amounts of certainmetallic ions, such as chromium, to the melt.

The invention will be more readily understood when taken in conjunctionwith the ydrawing which depicts the percent yield of yttrium-irongarnet, calculated by dividing the weight of garnet produced by thetotal weight of the melt, as a function of the initial composition ofthe lead oxide-lead iluoride flux. The numbered points represent dataobtained as described below.

A suitable procedure to be followed in the practice of this invention isoutlined below.

The reactants are weighed, mixed together, and placed in a platinumCrucible which is subsequently covered.

The cover preferably is crimped on to the crucible to avoid evaporationof the contents. The Crucible is then placed in a muilie furnace inwhich an oxygen-enriched atmosphere is maintained. The maintenance of anoxygen-enriched atmosphere decreases the tendency of molten lead oxideto reduce and attack the platinum Crucible.

1250 C., heating periods are typically in the range ot"- Vfromonetotwenty-four hours. The use Aof h1gher temperatures permits shorterheating times.

After fusion is complete, the meltsareV permitted to cool Vat a slowrate. Since equilibrium cooling is preferred, cooling should be `as slowas possi-ble. In general, cooling rates in the range of from 1 C. perhour to 10 C. per hour are advantageous although rates as high as 20 C.per hour may be used. After the crucible 1s cooled to a temperaturepreferably below 1.050 C., to assure high yields, it is removed from thefurnace and allowed to cool to room temperature. The solidified matrixis dissolved in a solvent such as a mixture of dilute nitric and aceticacids leaving the crystalline garnets unaffected.

The data shown in the drawing are presented to indicate the substantialincrease in yield of garnet single crystals which is made possible byuse of the inventive flux. Although the data relate to yttrium-irongarnet, it is to be understood that the advantages shown extend to theproduction of all garnet structures which require the use of a rareearth oxide as a starting material. ln the prior art production of suchgarnet structures, a limiting factor with respect to yield was the lowsolubility of the rare earth oxide reactant in the lead oxide ilux. Inthe present processes the addition of lead fluoride substantiallyincreases the solubility of the rare earth oxides and therebyfacilitates substantial increase in the yield.

In the drawing, point 1 represents the maximum percent yield ofyttrium-iron garnet which has lbeen produced in accordance with theprior art process using a ux of pure lead oxide. Under the conditionsused to obtain the data exemplilied by point 1, the melt was saturatedwith respect to yttrium oxide. The data with respect to point 1 wereobtained as follows:

Example I A mixture consisting of 7 grams YZOS, 70 grams Fe203,

and 100 grams of Pb() was heated to a temperature of V1350D C. for aperiod of hours and then cooled at a rate of approximately 5 C. per hourto a temperature of 970 C. The yield of yttrium-iron garnet wasdetermined to be 12.9 grams, thus the percent yield is equal to 12.9divided by 177, or ,approximately 7.4 percent. Also formed wereapproximately 30 grams of magnetoplumbite. Each of points 2 through 9represents a production run in which yttrium-iron garnet wascrystallized in a flux containing lead fluoride. The conditions underwhich the data for points 2 through 9 were obtained is set forth below,the number of the example corresponding to the number of the point. Itis to be noted that in all instances the maximum temperature was lessthan 1350 C., the temperature used in Example 1. Also, the temperaturesto which the melts were equilibrium cooled were greater than 970 C.which was the temperature employed in Example l. It is to be understoodthat such conditions eliminated temperature as a factor in comparingyields at different flux compositions.

Example 2 Also formed were approximately 22.3 grams of magneto plumbite.

Example 3 A mixture consisting of 20 grams Y2O3, 30 grams Fe203, 45grams PbF2, and 105 grams `of PbO was heated in a covered platinumcrucible to a temperature of 1250 C. in an oxygen-enriched atmospherefor a period of 4 hours and then cooled at a rate of approximately 20 C.per hour to a temperature of 990 C. The yield of yttrium-iron garnet wasdetermined to be 22.5 grams, and the percent yield is equal to 22.5divided by 200, or 11.2 percent. No magnetoplumbite was formed.

Example 4 A mixture consisting of 20 grams Y2O3, 30 grams Fe2O3, 60grams P-bFz, and 90 grams of PbO was heated in a covered platinumcrucible to a temperature of 1250J C. in an oxygen-enriched atmospherelfor a period of 4 hours and lthen cooled at a rate of approximately 2.3C. per hour to a temperature of 1000 C. The yield of yttrium-ironlgarnet was determined to be 23.1 grams, and the percent yieldis equalto 23.1 divided by 200, or 11.5 percent. No magnetoplumbite was formed.

Example 5 A mixture consisting` of 20 grams Y2O3, 30 grams Fe203, 75grams PbFg, and 75 grams of PbO was heated in a covered platinumcrucible to a temperature of 1250'J C. in an `oxygen-enriched atmospherefor a period of 4 hours and then cooled at a rate of approximately 20 C.per hour. to a temperature of 1000 C. The yield of yttrium-iron garnetwas determined to be 16.6 grams, and the percent yield is equal to 16.6divided by 200, or 8.3 percent. No magnetoplumbite was formed.

Example 6 A mixture consisting of 30 grams Y2O3, 45 grams Fe203, gramsPbFZ, and 70 grams of PbO was heated in a covered platinum crucible to atemperature of 1300 C. in an oxygen-enriched atmosphere for a period of4 hours and then cooled at a Irate of approximately 20 C. per hour to atemperature of 1000 C. The yield of yttrium-iron garnet was determinedto be 32.8 grams, and the percent yield is equal to 32.8 divided byV200, or 16.4 percent. Also formed were approximately 5.8 grams ofmagnetoplumbite.

Example 7 A mixture consisting of 20 grams Y2O3, 30 grams Fe203, gramsPbF2, and 50 grams of PbO was heated in a covered platinum crucible to atemperature of l250 C. lin an `oxygen-enriched atmosphere for a periodof 4 hours and then cooled at a rate'of approximately 2.3" C. per hourto a temperature `of 1000 C. The yield of yttrium-iron garnet wasdetermined to be 15.7 grams, and Ithe percent yield is equaly to V15.7divided by 200, or 7.8 percent. Also formed were approximately 7.5 gramsof magnetoplumbite.

Y Example 9 VA mixture consisting of 20 grams Y2O3, 30 grams FezQa, and|grams PbF2 was heated in a covered platinum crucible to a temperatureof 1`250 C. in `au oxygen-enriched atmosphere for a period of 4 hoursand Y then cooled at `a rate of approximately 2.3 C. ,per h our to atemperature of l000 C. The yield of yttrium-iron garnet was determinedto be 17.2 grams, thus the percent yield is equal to 17.2 -divided by200, or 8.6 percent. Also formed were approximately 12.1 grams ofmagnetoplumbite.

As can be seen from the drawing, each of points 2 through 9 representpercent yields which are greater than that of point 1. The yieldsrepresented by points 6 and 7 are approximately twice that of point 1.

It is believed that Example 9, the yield of which is plotted as point 9in the drawing, should be discussed more fully. Although the ux used wasinitially 100 percent lead fluoride, it has been determined thatsubstantial amounts of lead fluoride were converted to lead oxide in thecourse of the run. lt is believed that such conversion results frominteraction of the oxygen in the atmosphere with the lead iluoride flux.Accordingly, within a very short period of time following the inceptionof a production run, a flux which may have initially been pure leadiluoride is converted to a mixture of lead oxide and lead uoride.

Experimental data has also indicated that the yields of garnet arehigher at lower cooling rates. Thus, for example, increasing the coolingrate of Example 4 to C. per hour results in a decrease in yield ofapproximately percent. The yield of Example 5, which appears as adiscontinuity in the drawing, would be approximately equal to that ofExample 4 if the cooling rate were commensurately reduced. Accordingly,in those areas where the yield tends to be low, as, for example, at uxconcentrations of greater than 80 percent PbF2, a cooling rate of lessthan 5 C. per hour is preferred.

As stated above, one of the advantages of the use of the flux of thisinvention is the elimination of magnetoplumbite as a by-product in theproduction of yttriurniron garnet. It is noted that in Examples 3, 4 and5 above, no magnetoplumbite was formed. At lead liuoride concentrationsgreater than those employed in Examples 3, 4 and 5 the formation ofmagnetoplumbite may be avoided by adjusting the ratio of yttrium oxideto iron oxide in the melt so as to shift position on the phase diagramto a more desirable point. This was accomplished in the instance ofExample 10 set forth below.

Example 10 A mixture consisting of 22 grams Y203, 30 grams Fe203, 90grams PbF2, and 60 grams PbO was heated to a temperature ofapproximately 1300 C. for a period of 4 hours and then cooled at a rateof approximately 20 C. per hour to a temperature of 1005 C. The yield ofyttrium-iron garnet was found to `be 17 grams, and the percent yield wasequal to 8.4 percent. No maguetoplumbite was formed.

A comparison of Example 10 with Example 7 indicates that a sacrice inyield was necessary in order to eliminate the presence ofmagnetoplumbite. However, the reduced yield of Example 10 is stillgreater than that of Example 1, and the elimination of a separation stepmay compensate for the decrease in yield.

Deterioration of platinum crucibles is substantially lessened by use ofthe inventive ux. Such beneficial effect results directly from the lowertemperatures made possible by the addition of lead iluoridel to theprior art lead oxide flux. Operations may Ibe conducted at these lowertemperatures by reason of the increased solubility of the reactants inthe lead fluoride iiux. Thus, for example, the reactants used in Example1 above may be dissolved in an equal weight of ux containing 5 percentlead fluoride at a temperature approximately 50 C. lower than that whichwould be required if the ux were pure lead oxide. Such a decrease intemperature substantially lessens the severity of attack of the platinumcrucibles commonly used.

A 'substantial increase in the size of single crystal garnets is madepossible by the use of the inventive flux. For

instance, several single crystals of yttrium-iron garnet of the order of18 grams in weight were produced as described in Example 11, below. ltis noted that a run different from that of Example 11 only in that theflux was pure lead oxide resulted in crystals which were no larger than3 grams.

Example Il A mixture of 250 grams Y2O3, 375 grams Fe203, 800 grams PbO,and 1000 grams PbF2 was introduced into a cylindrical platinum crucibleapproximately 4 inches in height and 4 inches in diameter. The crucibleWas covered and was placed into a lux in which an oxygenenrichedatmosphere was maintained. The crucible and contents were heated to atemperature of approximately 1260 C. and maintained at that temperaturefor approximately 20 hours. The crucible and contents were then cooledat a rate of approximately 1/2 C. per hour to a temperature ofapproximately 1000 C. A temperature gradient -ot' approximately 10 C.was maintained between the top and bottom of the crucible, the top beingmaintained at the higher temperature.

The molten ux was decanted from the crucible, and the crucible andcontents then cooled to room temperature. The contents were then leachedwith a mixture of dilute nitric and acetic acids to dissolve theremaining flux. The largest single crystal produced was approximately 19grams -in weight. Several single crystals of the order of 18 grams inweight also resul-ted.

The lead oxide-lead fluoride ux of this invention may also be employedfor the production of single crystal garnets embodying elements otherthan yttrium, and also embodying yttrium in combination with such otherelements. Garnets may be produced in which yttrium is used incombination with rare earth elements of atomic number 57 through 71.Single crystal garnets embodying rare earth elements 62 through 71 orcombinations thereof may also lbe produced. Listed below are severalexamples of the production of such single `crystal garnets.

Example 12 lA mixture of 25 grams of erbium oxide (F1203), 60 gramsFe203, 60 grams of PbF2 and 90 grams PbO was heated in a coveredplatinum crucible at a temperature of 1300 C. for 5 hours in anoxygen-enriched atmosphere. The crucible was cooled at a rate of 1 C.per hour to a temperature of 890 C., and was then air-cooled -to roomtemperature. Single crystals of Er3Fe5O12 resulted.

Example 14 'A mixture of 25 grams of ytterbium oxide (YbzOa), 60 gramsFe203, 60 grams of PbF2 and 90 grams 'PbO was heated in a coveredplatinum crucible at a temperature of 1300 C. -for 5 hours in anoxygen-enriched atmosphere. The crucible was cooled at a rate of 1 C.per hour to a temperature of 890 C., and was then aircooled to roomtemperature. Single crystals of YbgFeOm resulted.

Example 15 A mixture of 20 grams Y2O3, 3'0 grams Fe2O3, 0.36

grams gadolinium oxide (GdzOa), grams of PbFZ and 60 grams PbO washeated in a covered platinum crucible at a temperature of 1260 C. for 8hours in an oxygenenriched atmosphere. The crucible was cooled at a rateof 1 C. per hour to a temperature of 930 C., and was Athen air-cooled toroom temperature.

Vparticular stone.

then air-cooled to room temperature.' Single crystals of (Y 99Gd01)3Fe5O12 resulted.

Example 16 A mixture of 20 grams YZOB, 30 grams FezO, 0.063 gramsholmium oxide (H0203), 90 gramsof PbF2 and 60 grams PbO was heated in acovered platinum crucible at a temperature of 1250 C. for 4 hours in anoxygenenriched atmosphere. The crucible was cooled at a rate of 1 C. perhour to a temperature of 985 C., and was Single crystals of (Y 99HO01)3F5O12 resulted.

The inventive flux is also suitable for producing substitutedyttrium-iron garnets of the type described in Example 17 A mixture of 20grams Y203, 26.25 grams Fe203, 3.25 grams Sc203, 90 grams PbF2, and 60grams PbO was heated to a temperature of approximately 1260 C. for aperiod of 8 hours and then cooled at a rate of approximately 1 C. perhour to a temperature of 930 C. Single crystals of scandium substitutedyttrium-iron garnet resulted.

A new series of synthetic gemstones have been produced using the flux ofthe present invention. These new gemstones are based on yttrium-galliumgarnet (Y3Ga5012). Single crystals of yttrium-gallium garnet arecolorless, transparent, and possess a refractive index of approximately1.82, and a hardness of between 7 and 8 on the Moh scale. The additionof small quantities of any of several metallic oxides imparts anattractive color to the garnet crystals. Thus, for example, the additionof chromium to this garnet produces a green gemstone, the addition ofcobalt produces a blue-green stone, and the addition of manganeseproduces a `stone having a ruby coloration. The gemstones exhibitrefractive indices as high as 1.85, depending upon the depth of color ofthe The hardness of the stones compares favorably with amethyst, and hasa value of between 7 to 8 on the Moh hardness scale. Examples of thevariety of gemstones, in addition to pure yttrium-gallium garnet, whichmay be produced in accordance withy this invention are set forth indetail below.

Example 18 A mixture consisting of 24 grams Y203, 48 grams Ga203, 180grams lPbFZ, 160 grams PbO, and 2.5 grams Cr203 was introduced into aplatinum crucible which was then covered and placed into a furnace inwhich an oxygen-enriched atmosphere was maintained. The crucible andcontents were heated to a temperature of 1250 C. andV maintained at thattemperature for approximately four hours. The Crucible and contents werethen cooled at a rateV of 'approximately'lo C. per hour to a temperatureof approximately 950 C. The Crucible was then allowed to cool to roomtemperature. The contents were then leached with a mixture of dilutenitric and acetic acids to dissolve the ilux and recover the garnetcrystals. The gemstones were green in color.

Example 19 The procedure of Example i11 was followed with the exceptionthat 0.116 gram of C0304 as added in place-of 2.5 grams of Cr203. Thegarnet crystals so formed were blue-green in color.

Example 20 The procedure followed was similar to that of Example 11Vwith the exception that 0.16 gram C0304 and 4 grams TiO2 was added inplace of 2.6 grams of Cr203. garnet crystals produced were teal-blue incolor.

The

Example 21 The procedure followed was similar to that of Example 11 withthe exception that 0.25 gram Mn203 was added in place of 2.5 grams ofCr203. The garnet crystals produced were ruby in color.

Example 22 Example 23 The procedure followed was similar to that ofExample 11 with the exception that 0.1 gram Fe203 and 0.5 gram C0304 wasadded in place of 2.5 grams of Cr203. The garnet crystals produced wereemerald-green in color.

Excessive addition of the certain metallic oxides may cause the garnetcrystals to become opaque, thereby depriving them of their gem-likequalities. In the instance of other metallic oxides the maximum amountswhich may be added are determined by other considerations. Listed intabular form below are the minimum and maximum limits of the additives,shown in Examples 18 through 23 above, to yield a garnet having gemlikeproperties. The minimum and maximum limits are expressed as a percentagecomputed by dividing the number of mols of additive by the sum of themols of additive plus mols of gallium oxide in the melt.

Table I Additive Minimum, Maximum,

percent percent 01203 1 12 C0304 001 2 TiOz 1 10 hlngOg 0l 3 iO 01 1F6203 01 1 Vwithout aiecting the gem-like qualities ofthe crystal.

However, it has been determined that a new phase or compound is formedbetween the Cr203 in excess of 12 percent and another ingredient of themelt. Accordingly, additions of Cr203 in excess of l2 percent may resultin a decrease of the total amount of yttrium-gallium garnet formed buthave no eiect on the coloration or appearance thereof.

The maximum limit in the instance of NiO represents Y the approximateupper limit of the solubility of NiO in the melt.

In the instances of Fe203 and Ti02 the maximum figures quoted representa practical upper limit in that additions in excess of these quantitiesdo not appreciably enhance the attractiveness of the garnet crystals.

It is to be appreciated that the examples set forth above are intendedmerely as illustrative of the advantages to be gained by the use of thelinx of this'invention. The number and variety of single crystalmaterials which may be produced are infinite. Variations may be made byone skilled in the art without departing lfrom the spirit and scope ofthis invention.

What is claimed is:

1. The method of producing single crystals of the gar- 9 net structurecomprising a compound represented by the formula AaB5012 Where A is atleast one element selected from the group con sisting of yttrium and therare earth elements of latomic number between 57 and 71 inclusive,

B is at least one metal selected from the group consisting of iron,aluminum, scandium, indium, gallium and chromium and O is oxygen,comprising the steps of forming a melt consisting essentially `of A, B,PbF2, together with lead oxide, the latter in an amount of from 0 to-400 weight percent based on the amount of PbFz present, `and coolingthe melt to produce single crystals.

2. The method of claim 1 in which A is yttrium, and in which the saidmelt consists essentially of Y2O3, PbO, PbF2, and at least one oxideselected from the group consisting of Fe203, A1203, Sc203, In2O3, Ga203,and Cr203.

3. The method of claim 1 in which A is yttrium,

B is iron, and in which the melt consists essentially of Y2O3, PbO,P-bFZ, and Fe203.

4. 'Ihe method of claim 3 in Which the Weight ratio of PbO to PbFz inthe melt is in the approximate range of hom 7:3 to 2:3.

5. The method of claim 3 in which the weight ratio of Fe203 to Y2O3 isapproximately 3:2 and the weight ratio of PbF2 to PbO is approximately5:4.

t6. The method of claim 1 in which Ais yttrium,

B is gallium, and in which the melt consists essentially of Y2O3, G3203,and PbF2.

7. The method of growing single crystal gems of the garnet structurecomprising the steps of forming a melt consisting essentially of Y2O3,Gat-203, PbO, PbF2, said PbO being present in an amount of from 0 to 400weight percent based on the amount of PbF2 present, and at least oneoxide selected from the group consisting of CI`203, C0304, Tioz, MD203,and F6203, and C001- ing the melt to produce single crystals.

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Anderson: J. Applied Physics, April 19159, p. 299S.

Gilleo et al.: Physical Rev., vol. 1110 (1958), pp. 73-8.

Bertaut et al.: Compt. Rend., vol. 243 (1956), pp. l219-22f

1. THE METHOD OF PRODUCING SINGLE CRYSTALS OF THE GARNET STRUCTURECOMPRISING A COMPOUND REPRESENTED BY THE FORMULA